We report the synthesis of polythiophene derivatives that contain decylthiophenyl side chains, using synthetic strategies that achieve high organic semiconductor Field effect mobilities by enhancing the pi-conjugation in the side-chain conjugated system. With increased charge densities, we obtained high field effect mobilities, LIP to 0.050 and 0.003 cm(2)/(V s) from poly(3,4"'-di(decylthiophenyl) sexithiophene) (PDTST) and poly(3,4"'-di(decylthiophenyl) quaterthiophene) (PDTQT), respectively. PDTQT and PDTST are characterized by UV-vis, DSC, A FM images, and G1XD patterns. Increased conformational rotation in the main backbone, caused by intramolecular repulsion between neighboring thiophene units, lowers the HOMO level and introduces remarkable chemical stability in the presence of air as well as increases processability due to high solubility. The high solubility and oxidative stability of PDTST and PDTQT indicate that these side-chain Conjugation system Strategies have the potential for improving other thiophene-based semiconducting polymers.